Some applications in an optical transport network require a very low differential latency between upstream, US, and downstream, DS, transmission via an optical transmission link comprising optical fibers. For example, in the common public radio interface CPRI, a very low differential latency is required. CPRI is a digital interface standard for encapsulating radio samples between a radio and a digital baseband processing unit. The CPRI interface is not packet-based. In the CPRI fronthaul, signals are multiplexed in a low latency time slot-like fashion. CPRI defines a maximum latency, a near-zero jitter and a near-zero bit error rate.
Accordingly, there is a need for a method and apparatus for measuring accurately a differential latency between an upstream and a downstream transmission performed via an optical transmission link.